John says Zircon. He too is surprised. I'm really vested in bubble free and easily polished glass for what it's worth these days using my own color work based in glasses that are going to show color well.
Trautman was certainly the voice in the wilderness on the Boro and I do get the feeling lots of times that in the other shops it's more the Tarot Card of the fool about which I could wax eloquent.
I had one guy who was making Hagy seals entirely wrong so they were meaningless, taking the seal and fusing it from top to bottom which tells you nothing but he was selling the color at $100lb . I pointed it out and he stopped getting pots from me.
It seems to me that if you can make 2800F, you should be able to melt boro batch. Croucher does it for Abe.